1. Field of the Invention
This invention relates to an improved method of manufacturing spiral tubes. Spiral tubes are normally used for applications in which a gas or fluid medium is circulated through the tubes and a gas or fluid medium circulated exterior of the tubes so as to exchange heat between the medium interiorly and the medium exteriorly of the tubes. Methods have been devised for manufacturing such spiraled tubes. However, the present methods are commercially applicable to tubes formed of a material such as aluminum or copper which is easily deformed by twisting to form a spiral. The present invention is directed towards an improved means of forming a spiral tube wherein the material of which the tube is formed is of a relatively harder, more brittle, and less ductile material.
2. Description of the Prior Art
For information relating to the method of manufacturing spiral tubes, reference may be had to previously issued U.S. Pat. Nos. 3,015,355; 3,533,267; and Re. 24,783. These prior issued U.S. patents show the concepts of forming spiral tubes of easily deformable, ductile material, such as aluminum, copper, and the like. The basic concept of manufacturing such spiral tubes such as set forth in Re. Pat. 24,783, has been improved as reflected in copending United States Pat. application No. 106,950, now Pat. No. 4,317,353, entitled: "TUBE TWISTING APPARATUS", and filed Dec. 26, 1979, by Elmo W. Geppelt et al.
These prior art devices work satisfactorily to produce spiral tubes; but when a tube is required to be of relatively hard, less ductile material, attempts to form spiral tubes utilizing the known technology have not been successful. When tubes must be formed of material capable of withstanding high temperatures, such as titanium, stainless steel and the like, particularly when the tube walls are relatively thick compared to tube diameters, the formation of a spiral groove by twisting a tube has not been successful. Instead of forming a spiral groove as the tube is twisted, the tubes have a tendency merely to twist in two. From experimentation it appears that a certain amount of ductility is required in order to extend a spiral throughout the required length of a tube by twisting the tube. When such ductility is not available in the material being used, the successful manufacture of spiral tubing utilizing the known prior art has not been commercially accomplished.
It is an object of this invention to provide an improved means of manufacturing spiral tubing of relatively nonductile material which overcomes the problems and limitations of the prior art.